Hierarchical Routing

In our study of LS and DV algorithm, we have looked at the network simply as a collection of interconnected routers. One router was indistinguishable from another in the sense that all routers executed the same routing algorithm to compute routing paths through the entire network. In fact, this model and its view of a homogenous set of routers all executing the same routing algorithm is a bit simplistic for at least two important reasons:

● Scale. As the number of routers becomes large, the overhead involved in computing, storing and communicating routing information (for instance, LS updates or least-cost path changes) becomes prohibitive. Today's public Internet consists of hundreds of millions of hosts. Storing routing information at each of these hosts would clearly require huge amounts of memory. The overhead required to broadcast LS updates among all of the routers in the public Internet would leave no bandwidth left for sending data packets. A distance-vector algorithm that iterated among such a large number of routers would surely never converge. Obviously, something must be done to reduce the complication of route computation in networks as large as the public Internet.

● Administrative autonomy. Although researchers tend to ignore issues such as a company's desire to run its routers as it pleases (for instance, to run whatever routing algorithm it chooses) or to hide aspects of its network's internal organization from the outside, these are important considerations. Ideally, an organization should be able to run and administer its network as it wishes, while still being able to connect its network to other outside networks.

Both of these problems can be solved by organizing routers into autonomous systems (ASs), with each AS consisting of a group of routers that are normally under the same administrative control (e.g., operated by the same ISP or belonging to the same company network). Routers within the same AS all run the same routing algorithm (for instance, an LS or DV algorithm) and have information about each other - exactly as was the case in our idealized model in the preceding section. The routing algorithm running within an autonomous system is called an intraautonomous system routing protocol. It will be necessary, certainly, to connect ASs to each other, and thus one or more of the routers in an AS will have the added task of being responsible for forwarding packets to destinations outside the AS: these routers are called gateway routers.

Figure 1 provides a simple example with three ASs: AS1, AS2, and AS3. In this figure, the heavy lines represent direct link connections between pairs of routers. The thinner lines hanging from the routers represent subnets that are directly connected to the routers. AS1 has four routers - 1a, 1b, 1c, and 1d - which run the intra-AS routing protocol used within AS1. Thus, each of these four routers knows how to forward packets along the optimal path to any destination within AS1. Likewise, autonomous systems AS2 and AS3 each have three routers. Note that the intra-AS routing protocols running in AS1, AS2, and AS3 need not be the same. Also note that the routers 1b, 1c, 2a, and 3a are all gateway routers.

It should now be clear how the routers in an AS determine routing paths for source-destination pairs that are internal to the AS. But there is still a big missing piece to the end-to-end routing puzzle. How does a router, within some AS, know how to route a packet to a destination that is outside the AS? It's easy to answer this question if the AS has only one gateway router that connects to only one other AS. In this case, because the AS's intra-AS routing algorithm has determined the least-cost path from each internal router to the gateway router, each internal router knows how it should forward the packet. The gateway router, upon receiving the packet, forwards the packet on the one link that leads outside the AS. The AS on the other side of the link then takes over the responsibility of routing the packet to its ultimate destination. As an example, assume router 2b in Figure 1 receives a packet whose destination is outside of AS2. Router 2b will then forward the packet to either router 2a or 2c, as specified by router 2b's forwarding table, which was configured by AS2's intra-AS routing protocol. The packet will finally arrive to the gateway router 2a, which will forward the packet to 1b. Once the packet has left 2a, AS2's job is done with this one packet.

So the problem is easy when the source AS has only one link that leads outside the AS. But what if the source AS has two or more links (through two or more gateway routers) that lead outside the AS? Then the problem of knowing where to forward the packet becomes considerably more challenging. For instance, consider a router in AS1 and suppose it receives a packet whose destination is outside the AS. The router should clearly forward the pack to one of its two gateway routers, 1b or 1c, but which one? To solve this problem, AS1 needs (1) to learn which destinations are reachable via AS2 and which destinations are reachable via AS3 and (2) to propagate this reachability information to all the routers within AS1, so that each router can configure its forwarding table to handle external-AS destinations. These two tasks - obtaining reachability information from neighboring ASs and propagating the reachability information to all routers internal to the AS - are handled by the inter-AS routing protocol. Since the inter-AS routing protocol involves communication between two ASs, the two communicating ASs must run the same inter-AS routing protocol. In fact, in the Internet all ASs run the same inter-AS routing protocol, called BGP4, which is discussed in the next section. As shown in Figure 1, each router receives information from an intra-AS routing protocol and an inter-AS routing protocol, and uses the information from both protocols to configure its forwarding table.

As an example, consider a subnet x (identified by its CIDRized address), and assume that AS1 learns from the inter-AS routing protocol that subnet x is reachable from AS3 but is not reachable from AS2. AS1 then propagates this information to all of its routers. When router 1d learns that subnet x is reachable from AS3, and hence from gateway 1c, it then determines, from the information provided by the intra-AS routing protocol, the router interface that is on the least-cost path from router 1d to gateway router 1c. Say this is interface I. The router 1d can then put the entry (x, I) into its forwarding table. (This example, and others presented in this section, gets the general ideas across but is a simplification of what really happens in the Internet. In the next section we'll provide a more detailed description, although more complicated, when we discuss BGP.)

Following up on the previous example, now assume that AS2 and AS3 connect to other ASs, which are not shown in the diagram. Also assume that AS1 learns from the inter-AS routing protocol that subnet x is reachable both from AS2, via gateway 1b, and from AS3, via gateway 1c. AS1 would then propagate this information to all its routers, including router 1d. In order to configure its forwarding table, router 1d would have to determine to which gateway router, 1b or 1c, it should direct packets that are destined for subnet x. One approach, which is often employed in practice, is to use hot-potato routing. ln hot-potato routing, the AS gets rid of the packet (the hot potato) as quickly as possible (more precisely, as inexpensively as possible). This is done by having a router send the packet to the gateway router that has the smallest router-to-gateway cost among all gateways with a path to the destination. In the context of the current example, hot-potato routing, running in 1d, would use information from the intra-AS routing protocol to determine the path costs to 1b and 1c, and then choose the path with the least cost. Once this path is chosen, router 1d adds an entry for subnet x in its forwarding table. Figure 2 summarizes the actions taken at router 1d for adding the new entry for x to the forwarding table.

When an AS learns about a destination from a neighboring AS, the AS can advertise this routing information to some of its other neighboring ASs. For instance,

assume AS1 learns from AS2 that subnet x is reachable via AS2. AS1 could then tell AS3 that x is reachable via AS1. In this manner, if AS3 needs to route a packet destined to x, AS3 would forward the packet to AS1, which would in turn forward the packet to AS2, As we'll see in our discussion of BGP, an AS has quite a bit of flexibility in deciding which destinations it advertises to its neighboring ASs. This is a policy decision, normally depending more on economic issues than on technical issues.

Remember that the Internet comprises a hierarchy of interconnected ISPs. So what is the relationship between lSPs and ASs? You might think that the routers in an ISP, and the links that interconnect them, form a single AS. Although this is often the case, many ISPs partition their network into various ASs. For instance, some tier-1 ISPs use one AS for their entire network; others break up their ISP into tens of interconnected ASs.

In brief, the problems of scale and administrative authority are solved by defining autonomous systems. Within an AS, all routers run the same intra-AS routing protocol. Among themselves, the ASs run the same inter-AS routing protocol. The problem of scale is solved because an intra-AS router need only know about routers within its AS. The problem of administrative authority is solved since an organization can run whatever intra-AS routing protocol it chooses; however, each pair of connected ASs needs to run the same inter-AS routing protocol to exchange reachability information. In the following section, we'll study two intra-AS routing protocols (RIP and OSPF) and the inter-AS routing protocol (BGP) that are used in today's Internet. These case studies will nicely round out our study of hierarchical routing.

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